The present invention relates to a reading apparatus for use in a facsimile equipment, a scanner, or the like.
Conventionally, a thin film transistor (TFT) matrix-driven image sensor, known as a contact-type image sensor, is used in a reading apparatus.
The image sensor is configured such that image information of an original or the like that is projected on a photo-detecting element array of the image sensor generates charges in the photo-detecting elements. The charges are temporarily stored in capacitors associates with the photo-detecting elements and the stored charges are read out in time series by using TFT switches. An equivalent circuit of an example of the image sensor is shown in FIG. 8. Photo-detecting elements are grouped into m blocks so that n photo-detecting elements are assigned to each block. That is, photo-detecting elements P.sub.11 -P.sub.1n constitutes a first block, photo-detecting elements P.sub.21 -P.sub.2n constitutes a second block. The same rule applies correspondingly to the following blocks and, finally, photo-detecting elements P.sub.m1 -P.sub.mn constitutes an m-th block. The respective outputs of the photo-detecting elements are connected to the drains of corresponding TFTs which are used as switching elements, that is, T.sub.11 -T.sub.1n, T.sub.21 -T.sub.2n, . . . , T.sub.m1 -T.sub.mn. The respective sources of the TFTs are connected to common output lines S.sub.1, S.sub.2, . . . , S.sub.n respectively so that the sources of the TFTs having the same order in the respective blocks are connected to one and the same output line. The respective gates of the switching elements T.sub.11 -T.sub.1n, T.sub.21 -T.sub.2n, . . . , T.sub.m1 -T.sub.mn are connected to a gate pulse generation circuit 14 through common signal lines G.sub.1, G.sub.2, . . . , G.sub.m provided for every block. There exist inter-line capacities C.sub.1, C.sub.2, . . . , C.sub.n in the common output lines S.sub.1, S.sub.2, . . . , S.sub.n, and those capacities are respectively connected to the drains of corresponding selection switches SW.sub.1, SW.sub.2, . . . , SW.sub.n each constituted by an FET. The sources of the selection switches SW.sub.1, SW.sub.2, . . . , SW.sub.n are commonly connected to an output line COM, and the gates of those switches are connected to a selection pulse generation circuit 15.
The switching operation will now be described. Gate pulse generation circuit 14 operates to turn-on the switching elements of a block. The output signals on the common output line are successively selected by the reading signal of the selection pulse generation circuit 15 in synchronism with generation of a gate pulse. For example, first, a gate pulse is outputted from the gate pulse generation circuit 14 onto the signal line G.sub.1. As a result, the switching elements T.sub.11 -T.sub.1n of the first block are turned on, so that the respective outputs of the photo-detecting elements P.sub.11 -P.sub.1n are connected to the common output lines S.sub.1, S.sub.2, . . . , S.sub.n and stored in the line capacities C.sub.1, C.sub.2, . . . , C.sub.n. The selection switches SW.sub.1, SW.sub.2, . . . , SW.sub.n are successively turned-on by the reading signal of the selection pulse generation circuit 15 to thereby output the output signals stored in the line capacities C.sub.1, C.sub.2, . . . , C.sub.n, that is, the outputs of the first block, onto the output line COM as a time-series signal. Next, a gate pulse is outputted from the gate pulse generation circuit 14 onto the signal line G.sub.2 so that the switching elements T.sub.21 -T.sub.2n of the second block are turned on to thereby connect the outputs of the photo-detecting elements P.sub.21 -P.sub.2n to the common output lines S.sub.1, S.sub.2, . . . , S.sub.n respectively. Then selection switches SW.sub.1, SW.sub.2, . . . , SW.sub.n are successively turned-on on the basis of the reading signal of the selection pulse generation circuit 15 to thereby output the outputs of the second block onto the output line COM as a time-series signal. The same rule applies correspondingly to the following till the outputs of the m-th block are outputted onto the output line COM as a time-series signal. Thus, image information for one line of an original is read. While performing sub-scanning, the above operation is repeated till the image information of the whole original has been read as a time-series signal.
As described above, in the reading apparatus in which the output of the photo-detecting array is read in matrix, there is a problem in the wiring of the output circuit. As seen from FIG. 8, crossing portions are formed in the output lines of the respective sources of the switching elements T.sub.11 -T.sub.1n, T.sub.21 -T.sub.2n, . . . , T.sub.m1 -T.sub.mn and the common output lines S.sub.1, S.sub.2, . . . , S.sub.n. Although at the crossing portions an insulating layer is interposed between the output lines, there has been a problem in that crosstalk may be generated between the signal lines by electrostatic capacities formed at the crossing portions so that charges cannot be read correctly.
As the method of solving this problem, there has been proposed such means in which a ground layer is interposed at the crossing portions where output lines cross in matrix, as disclosed, for example, in Japanese Patent Unexamined Publication Nos. Sho-64-5057 and Sho-64-5173. In such a method, however, there has been the disadvantage of an increase in the electrostatic capacities of the wiring, thus reducing sensitivity. Further, there has been the disadvantage of remarkable unevenness in crossing portions, thus a high yield in the production process cannot be achieved.